Electromagnetic impedance spectroscopy apparatus and related planar sensor system

ABSTRACT

According to various implementations, an apparatus for electromagnetic impedance spectrographic characterization of a material under test (MUT) includes: a planar array of at least two electrodes configured to be placed in electromagnetic communication with the MUT, wherein during operation of the planar array, at least one of the electrodes comprises a transmitting electrode for transmitting an electromagnetic signal over a range of frequencies through the MUT to at least one receiving electrode in the planar array; and a backer ground plate at least partially surrounding the at least two electrodes, the backer ground plate being electrically grounded and insulated from the at least two electrodes, wherein the backer ground plate extends from a plane formed by the at least two electrodes and separates the at least two electrodes to create an electrically isolated volume proximate to the at least two electrodes.

PRIORITY CLAIM(S)

This application claims priority to co-pending U.S. patent applicationSer. No. 17/669,687 (filed Feb. 11, 2022), which itself claims priorityto U.S. patent application Ser. No. 16/962,621 (now U.S. Pat. No.11,268,922, filed on Jul. 16, 2020), which itself claims priority toInternational Patent Application No. PCT/US2019/013936 (filed on Jan.17, 2019), which itself claims priority to U.S. Provisional PatentApplication No. 62/619,275 (filed on Jan. 19, 2018), the entire contentsof each of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

Aspects of the disclosure relate to electromagnetic impedancespectroscopy. In particular, aspects of the disclosure relate to anapparatus and related planar sensor system for use in characterizingselected portions of a material under test.

BACKGROUND

In U.S. Pat. No. 7,219,024 (which is hereby incorporated by reference inits entirety), a system is described for conducting electromagneticimpedance spectroscopy to non-invasively determine the in-place density(i.e., compaction) and moisture of various engineering materials withspecific interest in soils. A fundamental component of this conventionalsystem is the sensor system. Current testing standards for in-place soiltesting, such as ASTM standard D1556 (for sand cone method), ASTMstandard D6938 (for nuclear gauge method), and ASTM standard D7830 (forthe method described in U.S. Pat. No. 7,219,024) do not provide aspecific depth of soil to be tested. The physical design of theapparatus for the sand cone limits the testing depth to 4 to 6 inches,while the nuclear gauge provides for the placement of the radioactivesource to a depth of 10 to 12 inches. Both the sand cone method and thenuclear gauge method require invasive procedures. In order to match thedepth into the subsurface sampled by the accepted sand cone method, thesensor system disclosed in U.S. Pat. No. 7,219,024 must sample the soilto a depth of at least 4 inches below the surface. However, as notedherein, the nuclear gauge method provides for the placement of aradioactive source 10 to 12 inches into the soil. It may be beneficialfor the sensor assembly to sample to a depth of at least 6 inches.However, the signals received by the current art in sensor systems fromdepths exceeding 4 inches have an unacceptable level of noise, whichmakes it difficult to accurately measure the soil impedance anddetermine the soil properties at that depth.

BRIEF SUMMARY

All examples and features mentioned below can be combined in anytechnically possible way.

Aspects of the disclosure provide for an electromagnetic sensorapparatus which overcomes the limitations in the current art to increasethe depth of measurement into the surface and subsurface of a materialunder test (MUT) by reducing the noise of the received signals fromgreater depths. One implementation of the apparatus includes a sensorsystem including a transmitting electrode, a receiving electrode, and abacker ground plate that is configured to provide enclosed capacitivevolume(s) above the transmitting electrode(s) and enclosed capacitivevolume(s) above the receiving electrode(s). Various aspects focus onaxisymmetric electrodes as described in U.S. Pat. No. 7,219,024, butadditional aspects may utilize linear electrodes. The capacitive volumesmay contain air or some other dielectric material.

The objective of the enclosed capacitive volumes is to isolate andcontrol the effects of the field lines which emanate from both thetransmitting electrode and the receiving electrode and go to the backerground plate, and the field lines that pass through the soil and go tothe backer ground plate. One group of field lines are proximate thetransmitting electrode, transverse the soil at a shallow depth andinteract with the backer ground plate in the region of the transmittingelectrode. These field lines have greater strength (relative to otherlines described herein) and are minimally modified by their routethrough the soil. Field lines will also penetrate deeply into thesubsurface of the soil and interact with the backer ground plate in theregion proximate the receiving electrode, as well as with the receivingelectrode itself. These field lines will have less strength and are moreaffected (e.g., modified) by their route through the soil. In the sensorsystem of U.S. Pat. No. 7,219,024, the interaction of all of these fieldlines creates a varying field in the volume created by the electrodesand the intermediate ground. This varying field produces an uncontrolledparasitic capacitance that increases the noise level relative to thereceived signal, which inhibits accurate determination of the measuredimpedance. The creation of enclosed and separate capacitive volumes, thegeometry and volumes of which can be designed to control the degree ofthe parasitic capacitance, can significantly reduce this problem.

The preceding description concerns the electromagnetic sensor describedin U.S. Pat. No. 7,219,024 (hereby incorporated by reference in itsentirety), which has a single transmitting electrode and a singlereceiving electrode in an axisymmetric configuration. However, aspectsof the disclosure can be utilized to improve other sensor systems thathave a single transmitting electrode and multiple receiving electrodes,such as the systems described in U.S. Pat. No. 9,804,112 (e.g., at FIG.1 ). Additionally, aspects of the disclosure can be utilized to improvesensor systems having multiple transmitting and receiving electrodes,such as the systems described in U.S. Pat. No. 9,804,112 (e.g., at FIG.2 ) or US Patent Publication No. 2016/0161624 (e.g., at FIG. 10 ), bothof which are hereby incorporated by reference in their entirety.

Additional embodiments of the disclosure include use of a backer groundplate that is configured to provide enclosed capacitive volume(s) aboveeach of the transmitting electrodes and enclosed capacitive volume(s)above each of the receiving electrodes, in configurations having aplurality of such electrodes.

In particular implementations, a sensor system includes: a planar arrayof at least two electrodes configured to be placed in electromagneticcommunication with a material under test (MUT), wherein during operationof the planar array, at least one of the electrodes comprises atransmitting electrode for transmitting an electromagnetic signal over arange of frequencies through the MUT to at least one receiving electrodein the planar array; and a backer ground plate at least partiallysurrounding the at least two electrodes, the backer ground plate beingelectrically grounded and insulated from the at least two electrodes,wherein the backer ground plate extends from a plane formed by the atleast two electrodes and separates the at least two electrodes to createat least one electrically isolated volume proximate to the at least twoelectrodes.

In additional particular implementations, a sensor system includes: anapparatus for communicating with a material under test (MUT), theapparatus having: a planar array of at least two electrodes configuredto be placed in electromagnetic communication with the MUT, whereinduring operation of the apparatus at least one of the electrodescomprises a transmitting electrode for transmitting an electromagneticsignal over a range of frequencies through the MUT to at least onereceiving electrode in the planar array; and a backer ground plate atleast partially surrounding the at least two electrodes, the backerground plate being electrically grounded and insulated from the at leasttwo electrodes, wherein the backer ground plate extends from a planeformed by the at least two electrodes and separates the at least twoelectrodes to create at least one electrically isolated volume proximateto the at least two electrodes; and a circuit coupled with theapparatus, the circuit configured to transmit a set of electromagneticsignals through the planar array to the MUT, and receive at least onereturn signal from the planar array.

In some cases, the backer ground plate is formed of an electricallyconductive material.

In particular aspects, the backer ground plate includes a set ofrecesses corresponding with each of the at least two electrodes.

In certain implementations, the plane formed by the at least twoelectrodes is substantially parallel with a surface of the MUT.

In some aspects, the array of at least two electrodes includesaxisymmetric circular electrodes.

In certain cases, the backer ground plate extends between neighboringelectrodes in the array to the plane formed by the at least twoelectrodes.

In particular implementations, a first one of the at least twoelectrodes includes a central electrode and a second one of the at leasttwo electrodes includes an outer electrode.

In some cases, the central electrode includes the transmittingelectrode.

In certain aspects, the outer electrode includes the transmittingelectrode.

In particular cases, the planar array of at least two electrodes isarranged in a linear configuration.

In some implementations, each of the at least two electrodes has arectangular shape with rounded corners.

In certain implementations, any single electrode in the array includesthe transmitting electrode and remaining electrodes in the arrayincludes the at least one receiving electrode.

In particular cases, the transmitting electrode is selectable among theelectrodes of the array, and is configured to change between theelectrodes of the array.

In some aspects, the array includes a plurality of transmittingelectrodes and a plurality of receiving electrodes.

In certain implementations, during operation of the apparatus, theplanar array is in direct physical contact with the MUT and iselectrically non-conductive with the MUT.

In particular aspects, the circuit includes a signal analyzer configuredto compare the set of electromagnetic signals with the at least onereturn signal.

In some cases, the circuit further includes a computing device coupledwith the signal analyzer, where the computing device is configured todetermine a characteristic of the MUT based upon the comparison of theset of electromagnetic signals with the at least one return signal.

In certain aspects, determining the characteristic of the MUT includes:determining a difference in an aspect of the at least one return signaland an aspect of the set of electromagnetic signals; comparing thedifference in the aspect to a predetermined threshold; and determiningthe characteristic of the MUT based upon the compared difference.

In some cases, during operation of the system, the planar array is indirect physical contact with the MUT and is electrically non-conductivewith the MUT.

In certain aspects, the conductive backer ground plate extends betweenneighboring electrodes in the sensor array and defines a set of separatevolumes corresponding with each of the at least two electrodes.

In particular implementations, the plane formed by the at least twoelectrodes and the conductive backer ground plate is substantiallyparallel with a surface of the MUT.

In some aspects, the array of at least two electrodes includesaxisymmetric circular electrodes, where a first one of the at least twoelectrodes includes a central electrode and a second one of the at leasttwo electrodes includes an outer electrode.

In certain implementations, the central electrode includes thetransmitting electrode and wherein the outer electrode includes thereceiving electrode.

In particular aspects, the central electrode includes the receivingelectrode and wherein the outer electrode includes the transmittingelectrode.

In some implementations, the planar sensor array is arranged in a linearrectangular configuration, where a first one of the at least twoelectrodes is a transmitting electrode and a second one of the at leasttwo electrodes is a receiving electrode.

In certain aspects, a spacing of the conductive backer ground platebetween the transmitting electrode and receiving electrode is sized toadjust for a depth of penetration and measurement into the MUT.

In particular implementations, each of the at least two electrodes has arectangular shape with rounded corners.

In some cases, the array of at least two electrodes includes at leastthree electrodes arranged in a linear rectangular configuration.

In certain implementations, a single electrode in the planar sensorarray includes the transmitting electrode and the remaining electrodesin the planar sensor array include the at least one receiving electrode.

In particular aspects, the planar sensor array includes a plurality oftransmitting electrodes and a plurality of receiving electrodes.

In some cases, the transmitting electrode is selectable among theelectrodes of the planar sensor array, and is configured to changebetween the electrodes of the planar sensor array.

In certain implementations, during operation of the apparatus, theplanar sensor array is in direct physical contact with the MUT and iseither electrically non-conductive with the MUT or electricallyconductive with the MUT.

In particular cases, the circuit includes a signal generator operatingover a range of frequencies for transmitting the electromagnetic signalfrom the transmitting electrode of the planar sensor system through theMUT and a signal analyzer configured to analyze the at least one returnsignal.

In some aspects, the circuit further includes a computing device coupledwith the signal analyzer, where the computing device is configured todetermine a physical property of the MUT based upon the transmittedelectromagnetic signal and the at least one return signal.

In particular implementations, determining the physical property of theMUT includes: transmitting the electromagnetic signal over a range offrequencies from the transmitting electrode through the MUT to thereceiving electrode; comparing the transmitted electromagnetic signal tothe received signal to compute an electromagnetic property of the MUT;and correlating the computed electromagnetic property of the MUT with aphysical property of the MUT to determine the physical property of theMUT.

In certain aspects, during operation of the system, the planar sensorarray is in direct physical contact with the MUT and is eitherelectrically non-conductive with the MUT or electrically conductive withthe MUT.

In some cases, the conductive backer ground plate extends betweenneighboring electrodes in the sensor array in the plane of theelectrodes and encompasses the electrodes to create a set of separatevolumes corresponding with each of the at least two electrodes with thevolumes for each electrode being individually optimized for minimalparasitic impedance.

In particular implementations, the plane formed by the at least twoelectrodes and the conductive backer ground plate is substantiallyparallel with a surface of the MUT.

In certain aspects, the array of at least two electrodes includesaxisymmetric circular electrodes, wherein a first one of the at leasttwo electrodes comprises a central electrode and a second one of the atleast two electrodes comprises an outer electrode.

In some implementations, the array of at least two electrodes comprisesaxisymmetric circular electrodes, wherein the central electrodecomprises the transmitting electrode and wherein the outer electrodecomprises the receiving electrode.

In particular cases, the central electrode comprises the receivingelectrode and the outer electrode comprises the transmitting electrode.

In some aspects, the planar array of two electrodes is arranged in alinear rectangular configuration with one being a transmitting electrodeand one being a receiving electrode.

In certain implementations, the spacing of the conductive backer groundplate between the transmitting electrode and receiving electrode issized to adjust for the depth of penetration and measurement into theMUT.

In particular aspects, each of the at least two electrodes has arectangular shape with rounded corners.

In some cases, the planar array of at least three electrodes is arrangedin a linear rectangular configuration.

In certain aspects, any single electrode in the array comprises thetransmitting electrode and remaining electrodes in the array comprisethe at least one receiving electrode.

In particular implementations, the array comprises a plurality oftransmitting electrodes and a plurality of receiving electrodes.

In some aspects, the transmitting electrode is selectable among theelectrodes of the array, and is configured to change between theelectrodes of the array.

In particular cases, the circuit includes a signal generator andanalyzer operating over a range of frequencies transmitting theelectromagnetic signal from the transmitting electrode of the planarsensor system through the MUT and configured to receive the returnsignal to analyze the at least one return signal.

In certain implementations, the circuit further includes a computingdevice coupled with the signal generator and analyzer, wherein thecomputing device is configured to compute an electromagneticcharacterization of the MUT and to correlate the computedelectromagnetic characterization of the MUT to a physical property ofthe MUT.

In particular aspects, determining a physical characteristic of the MUTcomprises: determining the electromagnetic characteristic of the MUT bytransmitting an electromagnetic signal over a range of frequencies fromthe planar sensor system transmitting electrode through the MUT to thereceiving electrode; comparing the transmitted signal to the receivedsignal to compute an electromagnetic property of the MUT; developing analgorithm to correlate the computed electromagnetic property of the MUTwith a physical property of the MUT; and applying the algorithm to ameasured electromagnetic characteristic to determine the physicalproperty.

Two or more features described in this disclosure, including thosedescribed in this summary section, may be combined to formimplementations not specifically described herein.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features, objectsand benefits will be apparent from the description and drawings, andfrom the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of this disclosure will be described in detail, withreference to the following figures, where like designations denote likeelements, and where:

FIG. 1 shows a cross-sectional view of a prior art sensor.

FIG. 2 shows a cross-sectional view of computed field lines associatedwith the prior art sensor of FIG. 1 .

FIG. 3 shows a cross-sectional view of an apparatus according to variousembodiments of the disclosure.

FIG. 4 shows a plan view of apparatus of FIG. 3 .

FIG. 5 shows a block diagram of system according to various embodimentsof the disclosure.

FIG. 6 shows a cross-sectional view of an apparatus according to variousadditional embodiments of the disclosure.

FIG. 7 shows a plan view of the apparatus of FIG. 6 .

FIG. 8 shows a cross-sectional view of computed field lines associatedwith the apparatus of FIG. 6 .

FIG. 9 shows a cross-sectional view of an apparatus according to variousfurther embodiments of the disclosure.

It is noted that the drawings of the various implementations are notnecessarily to scale. The drawings are intended to depict only typicalaspects of the disclosure, and therefore should not be considered aslimiting the scope of the implementations. In the drawings, likenumbering represents like elements between the drawings.

DETAILED DESCRIPTION

As described herein, various aspects of the disclosure include systemsfor characterizing a material under test (MUT). In particular, sensorsystems disclosed according to various embodiments include a backerground plate which enclose the electrodes to create volumes forcontrolling the parasitic impedance that can cause signal interferencein characterization of a MUT.

According to various embodiments, an MUT can include any materialcapable of being characterized via one or more approaches shown and/ordescribed herein. In various embodiments, an MUT includes an inorganicmaterial such as a soil, an organic material such as grain, or abiological material such as tissue, sub-tissue, organs, fluids, etc. AnMUT can include synthetic, composite and/or other blended/modifiedmaterials. An MUT can also include elemental materials, as well asmaterials including impurities. It is understood that the teachingsdescribed according to the various embodiments herein can be applied toany MUT described herein, as well as other materials that can becharacterized according to the approaches of the various embodiments.

In the following description, reference is made to the accompanyingdrawings that form a part thereof, and in which is shown by way ofillustration specific exemplary embodiments in which the presentteachings may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice thepresent teachings and it is to be understood that other embodiments maybe utilized and that changes may be made without departing from thescope of the present teachings. The following description is, therefore,merely exemplary.

A prior art sensor configuration in U.S. Pat. No. 7,219,024 is shown inFIG. 1 . As shown, a sensor 20 includes an active inner (transmitting)electrode 100, a ground plane 102, and then one or more sense(receiving) electrodes 104. Sensor 20 may also include a guard element(or, guard ring) electrode 106. As shown in FIG. 1 , electromagneticfield lines 107 and 108 are created in the material under test (MUT) 110by applying an electromagnetic signal to the transmitting electrode 100.The electromagnetic field lines 107 are transmitted from transmittingelectrode 100 through the MUT 110 to the ground plane 102. Theelectromagnetic field lines 108 are transmitted from transmittingelectrode 100 through the MUT 110 to receiving electrode(s) 104. Asdescribed in U.S. Pat. No. 7,219,024, an advantageous function (relativeto the then conventional approach) of the ground plane 102 is tointercept the electromagnetic field 107 from the receiving electrode 104and permit the electromagnetic field 108 that penetrates into deeperportions of the MUT 110 to be measured by the receiving electrode(s)104. This may provide two advantages: first, a reduced sensitivity toimperfections on the surface of material 110; and second, a measurementof a deeper penetration of the electromagnetic field 108 and hence, adeeper measurement depth into the subsurface of the MUT. Sensor 20 maybe in contact with the MUT 110 during use or separated from the MUT 110by a small air gap. The signal received by the receiving electrode(s)104 is altered by the impedance characteristics of the MUT 110. Guardelement 106 is described as being driven by an electrical potentialsubstantially equal to that present on receiving electrode(s) 104, andis able to substantially reduce interaction of the electromagnetic field108 with other external sources of electromagnetic interaction.

However, in practice, a problem exists with keeping the guard element106 at the same electric potential as the receiving electrode(s) 104.The field at the receive electrode 104 varies with the type of MUT beingmeasured. The magnitude of the field is determined only upon measurementof the MUT 110, and cannot be determined a priori. Further, there is aphase shift in the signal, which also varies with the type of MUT beingmeasured. As with magnitude, this phase shift cannot be determined apriori. Further, both the magnitude and phase shift vary with thefrequency of the transmitted signal. Thus, while the goal of driving theelectric potential of the guard element 106 to be substantially equal tothat of the receiving electrode(s) 104 is theoretically desirable, it isimpossible in practice. In practice, the potential of the guard element106 is allowed to float.

The net effect of this configuration is that the four electrodeelements, the transmitting electrode 100, the receiving electrode 104,the ground plane electrode 102, and the floating guard element (or,ring) electrode 106 are all at different potentials, due to the varyingmagnitude of the field lines interacting with the various electrodes,and the different phases of the signal of those field lines. Using theprior art configuration shown in FIG. 1 , field lines were computedusing COMSOL Multiphysics®, a software platform available from COMSOL,Inc., of Stockholm, Sweden, in accordance with various implementationsdisclosed herein. The results of this field line computation are shownin FIG. 2 . As shown in FIG. 2 , field lines 107 pass through MUT 110from the transmitting electrode 100, and go to the ground plane 102,illustrated in FIG. 1 . FIG. 2 also illustrates the field lines 108 thatpass through the MUT 110 from the transmitting electrode 100 and arereceived at the receiving electrode 104. However, there are additionalflows of the field not shown in FIG. 1 that affect the ability of thesensor system to secure accurate reading of the impedance of the MUT110. First, there is a group of field lines 131 that travel from thetransmitting electrode 100 to the ground plane 102. Additionally, thereare field lines 132 that travel from the receiving electrode 104 to theguard element (or, floating guard ring) electrode 106. Finally, thereare field lines 133 that travel from the guard element electrode 106 tothe ground plane 102. The only potentials that can be reliably known inthis situation are the potential of the signal sent to the transmittingelectrode and the ground plane. Due to attenuation (i.e., reduction inmagnitude) of the signal from the transmitting electrode 100, as well asa phase shift of that signal as it passes through the MUT 110 to thereceiving electrode 104 (along with variation of these effects with thefrequency of the transmitted signal), the quantities of the magnitudeand phase of the received signals can be difficult to discern. Inaddition, there are electromagnetic fields flowing between the receivingelectrode 104 and the guard element electrode 106, and between the guardelement electrode 106 and the ground plane 102. These fields affect theelectric potential of the guard element electrode 106, which in turnaffects the effective potential of the receiving electrode 104. As thesignals can vary in magnitude and phase, the precision of themeasurement of the impedance of the MUT 110 can become degraded in somecircumstances. The relative effect of the degradation in signalprecision varies with frequency, because as the frequency increases, theattenuation and phase shift of the signal passing through the MUT 110increases.

According to various implementations, an apparatus 300 (e.g., includingan axisymmetric planar sensor, also referred to as a sensor array) isconfigured to effectively characterize the MUT 110 as shown in theschematic cross-sectional depiction of FIG. 3 . Apparatus 300 addressesvarious shortcomings of the conventional MUT characterization systems.As illustrated in the plan view of the apparatus 300 in FIG. 4 , theapparatus 300 has an axisymmetric planar electrode configuration, with atransmitting electrode (TX) 301 at its radial center. A receivingelectrode (RX) 304 surrounds the transmitting electrode 301. In variousimplementations, a conductive backer ground plate (or, backer groundplate) 314 is located both between the transmitting electrode 301 andthe receiving electrode 304, and surrounding the receiving electrode304, and extends from a plane 305 formed by the at least two electrodesand separates the electrodes 301, 304 to create an electrically isolatedvolume proximate each of the electrodes 301, 304. In variousembodiments, the plane 305 is substantially parallel with a surface ofthe MUT 110 (e.g., within a margin of approximately +/−1-3%) throughwhich the electromagnetic signals pass.

Backer ground plate 314 can be formed of an electrically conductivematerial. According to particular example embodiments, the backer groundplate 314 is formed of conductive material such as a metal (e.g.,aluminum or steel), while the electrodes 301, 304 are formed of a metalsuch as copper. In some cases, the backer ground plate 314 can extendbetween neighboring electrodes (e.g., electrodes 301, 304) to the plane305 formed by those electrodes. That is, the backer ground plate 314 canlaterally separate the neighboring electrodes across an entire depth ofthose electrodes (e.g., as measured from plane 305). The geometry of thebacker ground plate 314 can be configured to surround the transmittingelectrode 301 and the receiving electrode 304 and create enclosedcapacitive volumes (VR) 315, (VT) 316 over both of those electrodes(FIG. 3 ). These capacitive volumes 315, 316 are physically isolatedfrom each other by the backer ground plate 314. In some cases, thecapacitive volumes 315, 316 include recesses corresponding with each ofthe electrodes 301, 304, and are sealed by the electrodes 301, 304(including an insulating material between each capacitive volume 315,316 and the electrodes 301, 304).

In various implementations, the backer ground plate 314 permits controlof the impedance of the capacitive volumes 315 and 316 between theelectrodes 301 and 304 and the backer ground plate 314. By appropriatelysizing the capacitive volume(s), the parasitic impedance of the volumes,YR and VT, may be reduced, which can significantly increase theprecision of the signal measured at the receiving electrode 304. Thecapacitive volumes, YR and VT, can be sized separately based on theinteractions between the sensor geometry and the interaction with theMUT 110. The primary design variable is the distances between the backof the electrodes 301, 304 and the top inside surface of the conductiveground plane backer 314, noted as dR and dT in FIG. 3 . These distancesand volumes can be sized separately to enhance the performance of eachelectrode 301, 304 in detecting impedance characteristics of the MUT110.

As noted herein, the apparatus 300 can include the array of electrodes301, 304 (or additional electrodes, as discussed herein) forcommunicating (non-conductively) with a surface and a subsurface beneaththe surface of the MUT 110. In particular embodiments, during operationof the apparatus 300, the electrodes 301, 304 can be located in directcontact with the MUT 110. In these embodiments, electrodes 301, 304 areplaced in non-conductive (electrical) contact with the MUT 110, suchthat electrical current does not flow from electrodes 301, 304 to theMUT 110. In other embodiments, electrodes 301, 304 are placed proximateto the MUT 110, but not in physical contact with the MUT 110. In thesecases, for a given electrode geometry, placing the electrodes 301, 304at a distance from the surface of the MUT 110 affects the penetration ofsignals into the surface and sub-surface of the MUT 110, and thestrength and phase of the signal received at the correspondingelectrode(s). In any case, the distance between the electrodes 301, 304and the surface of the MUT 110 can be factored into the calculation ofthe impedance characteristics of that MUT 110 as derived from thetransmitted and received signals.

FIG. 4 is a plan view of the apparatus 300, including the transmittingelectrode 301 and receiving electrode 304, with the backer ground plate314 interposed between the two electrodes 301, 304. In this exampleimplementation, the electrodes 301, 304 and the backer ground plate 314are circular, and aligned symmetrically about a central axis (L) (FIG. 3). However, it is understood that this alignment and the shaping ofelectrode 301, 304 is only one example of various possibleconfigurations for apparatus 300. As described herein, apparatus 300 canbe configured, modified, etc., to include a linear array of electrodes,or in other cases, may include an arced array of electrodes, a multi-rowarray of electrodes, or any other configuration of electrodes describedin the '112 patent (incorporated by reference herein).

In various additional implementations, a system 500 is disclosedincluding the apparatus 300 of FIGS. 3 and 4 coupled with a circuit 501,as shown in the schematic depiction of FIG. 5 . Circuit 501 can beelectrically coupled with electrodes 301, 304 in order to performvarious processes in characterizing the MUT 110. Circuit 501 can beconfigured to: supply a transmitting signal to the transmittingelectrode 301 (e.g., over a range of frequencies), obtain a receivingsignal via the receiving electrode 304, compare the received signal witha reference signal to determine a change in an aspect of that signal(e.g., magnitude change and/or phase shift) from the transmittingelectrode 301 to the receiving electrode 304, compute the impedance ofthe MUT 110 (e.g., using a computing device 510), and determine at leastone physical property of the MUT 110 based upon the computed impedance.In various implementations, computing device 510 can include aprogrammable controller or other conventional control logic orarchitecture for initiating signal transmission, reception andcalculation of characteristics of the MUT 110.

Circuit 501 can further include a signal generator 520, which caninclude a sine wave signal generator such as a direct digitalsynthesizer (DDS) with a dual output (e.g. such as AD9958, availablefrom Analog Devices, of Norwood, Mass.). The signal generator 520 canproduce a signal over a range of frequencies appropriate for theimpedance spectroscopy evaluation of the MUT 110. The raw signal fromthe signal generator 520 is processed by an operational amplifier 521(e.g. such as AD8001, available from Analog Devices, of Norwood, Mass.)to convert the signal to a single output and ground. The output ofoperational amplifier 521 is split with a first leg 525 going to thetransmitting electrode 301 and a second leg 527 going as the referencesignal to a signal analyzer 522. The ground 526 of operational amplifier521 is connected to the backer ground plate 314. The return signal 524from the receiving electrode 304 is transmitted to the analyzer 522,where it is compared to the reference signal 527 from the signalgenerator 520 to determine the magnitude change and phase shifts betweenthe return signal 524 and the reference signal 527. These quantities aretransmitted to the computing device 510 for computing the impedance ofthe MUT 110 and correlation of the signal characteristics with physicalproperties of the MUT 110.

While the example configuration of apparatus 300 includes axisymmetricelectrodes, it is understood that according to various implementations,the electrodes may be linear rather than axisymmetric. Linear electrodescan offer benefits, e.g., reduction in certain dimensions of the sensorto achieve a desired depth of measurement into the subsurface of the MUT110, or isolation of impedance measurements for particular voxels (or,sub-volumes) of the MUT 110. As is known in the art, the depth ofpenetration of the electromagnetic field into the MUT is determined bythe distance between the transmitting electrode and the receivingelectrode(s). In an axisymmetric sensor design such as that shown inapparatus 300, the depth of penetration of the electromagnetic fieldinto the MUT 110 is determined by the radius of the receiving electrode.This is illustrated, for example, in FIGS. 2 and 8 . Therefore, a linearelectrode design can provide for a deeper penetration into the MUT witha smaller overall size than the axisymmetric sensor. Some aspects oflinear electrode configurations are described in U.S. Pat. No.9,804,112, which is hereby incorporated by reference in its entirety.FIGS. 6 and 7 present illustrations of a cross-sectional side view, andplan view, respectively, of an electromagnetic impedance sensorapparatus (sensor apparatus) 700 with an array of linearly arrangedelectrodes, according to various embodiments. This sensor apparatus 700can include a set of linearly arranged electrodes, including atransmitting electrode 701 and a receiving electrode 702 aligned withinthe same geometric plane 705 with one another, but separated by aconductive backer ground plate (backer ground plate) 714 (similarly tothe apparatus 300 of FIGS. 3-5 ). Backer ground plate 714 can isolatethe transmitting electrode 701 and receiving electrode 702, formingdistinct capacitive volumes (VT) 703 and (VR) 704, in a similar manneras described with respect to apparatus 300. In use, the electromagneticfield lines in apparatus 700 will be similar to those shown in the fieldline diagram of FIG. 8 with the field lines going from the transmittingelectrode (301 or 701) on the left to the receiving electrode (304 or702) on the right.

As described herein, in various implementations of the sensor apparatus700, the backer ground plate 714 permits control of the impedance of thecapacitive volumes 703 and 704 between the electrodes 701 and 702 andthe backer ground plate 714. By appropriately sizing the capacitivevolume(s), the parasitic impedance of the volumes VR and VT, may bereduced, which can significantly increase the precision of the signalmeasured at the receiving electrode 702. The capacitive volumes, VR andVT, are sized separately based on the interactions between the sensorgeometry and the interaction with the MUT 110. In these exampleimplementations, the primary design variable is the distance(s) betweenthe back of the electrodes 701 and 702 and the top inside surface of theconductive backer ground plate 714, noted as dR and dT in FIG. 6 . Thesedistances and volumes can be sized separately to enhance the performanceof each electrode 701, 702 in characterizing physical properties of theMUT 110.

The sizes and spacing of the electrodes 701 and 7012 shown in FIG. 7helps to determine the depth that the electromagnetic field line(s) willpenetrate into the MUT 110, and where in the MUT 110 measurements of theimpedance characteristics of the MUT 110 can be performed. This isillustrated and discussed further in the following sections. Referringto FIG. 7 , the length of the transmitting electrode 701 is designatedas L₁ and the length of the receiving electrode 702 is designated as L₃.The spacing between the electrodes in which the conductive backer groundplate 714 extends (along the plane of the electrodes, illustrated asplane 705 in FIG. 6 ) is designated as L₂. There is an electricalinsulating barrier between the electrodes and the backer ground platewhich is shown as 706 in FIGS. 6 and 7 . The separation L₂ between thetransmitting electrode 701 and the receiving electrode 704 causes theelectromagnetic field lines that traverse the shallower portions of theMUT 110 to be intercepted by the ground plane 714 before they reach thereceiving electrode 704. This causes only the electromagnetic fieldlines that traverse the MUT 110 at greater depths to be sensed at thereceiving electrode 704. This is illustrated in the depiction of fieldlines traversing the MUT 110 as illustrated in FIG. 8 and discussedherein.

As with the computation performed with respect to FIG. 2 , COMSOLMultiphysics® was used to compute the field lines of the sensorapparatus 700 in practice. FIG. 8 shows a schematic depiction of thesensor apparatus 700, including depiction of the field lines producedusing this example test. Field lines 632 are shown travelling from thetransmitting electrode 701 through the MUT 110 to the backer groundplate 714. Additional field lines 631 are also shown travelling directlyfrom the transmitting electrode 701 the backer ground plate 714 acrossthe gap, dT (FIG. 6 ). Additionally, field lines 633 are showntravelling from the transmitting electrode 701 through the MUT 110 tothe receiving electrode 704. These field lines 633 may be of greatestsignificance, as they penetrate the MUT 110 and carry the desiredinformation relating to the impedance and/or other physical propertiesof the MUT 110. The depth to which the field line(s) 633 penetrate intothe subsurface of the MUT 110 is primarily a function of the spacing(L₂) between the transmitting electrode 701 and receiving electrode(s)704, (FIG. 7 ). The placement of the backer ground plate 714 between(e.g., interposed within the same geometric plane 705 as the electrodes)the transmitting electrode 701 and the receiving electrode 704intercepts field lines that traverse near the surface 640 of the MUT110, and reduces the impact those field lines would otherwise have onskewing the measurement of impedance and/or other characteristics of theMUT 110 to a volume of the subsurface that is not of interest. Becausethe backer ground plate 714 provides a constant electric potential, asnoted herein, the interference between the receiving electrode 704 andthe backer ground plate 714 is minimized. This can improve the precisionof the measurement when compared with conventional systems.

U.S. Pat. No. 9,804,112 (e.g., in FIG. 2 ) and US Patent Publication No.2016/0161624 (e.g., in FIG. 13 ) present sensor arrays with multipleelectrodes. FIG. 9 illustrates one example embodiment of an apparatusincluding a multiple-linear electrode configuration. In this case, anapparatus 900 is shown including a plurality of linearly arrangedelectrodes. This apparatus 900 includes four linearly disposedelectrodes 901(E1), 902(E2), 903(E3), and 904(E4). A conductive backerground plate (backer ground plate) 917 is shown as having fourconductive volumes, 905(V1), 906(V2), 907(V3), and 908(V4). For purposesof illustration, any of the four electrodes E1-E4 may be a transmittingelectrode and the remaining three may be receiving electrodes. Forexample, where electrode 901 (E1) is the transmitting electrode, thereceiving electrodes (902, 903 and 904) will receive the field linesfrom different volumes of the MUT 110. The geometry of the field linesin this example configuration can be similar to those shown in theexample depiction in FIG. 6 . In any case, the backer ground plate 917creates isolated capacitive volumes, 905(V1), 906(V2), 907(V3), and908(V4), and creates a uniform voltage potential for all of theelectrodes. When compared with conventional systems, these isolatedvolumes and uniform voltage potential can enhance the accuracy ofimpedance readings, e.g., of the induced magnitude attenuation and phaseshift of the transmitted signal as it passes through the MUT 110.

While FIG. 9 illustrates an electromagnetic impedance sensor apparatus900 with four electrodes, the actual number of electrodes may be limitedonly by the strength of the transmitted signal, the depth into thesubsurface of the MUT 110 desired to be measured, the number of voxelsand sub-voxels desired to be measured, and the ability to measure theattenuated receive signals. As such, any number of electrodes could bedeployed using the sensor configuration of apparatus 900 (or any othersystems shown and described herein).

The design of the individual electrodes in the various arrays discussedwith reference to one or more FIGURES may be circular in shape. However,in some embodiments, a circular-shaped electrode may limit the potentialof field concentration available if the desired area of detection in theMUT included a corner or a point. In various embodiments, at least oneof the electrodes has an ellipsoid shape. In various other embodiments,as noted herein, at least one of the electrodes has a rectangular shapewith rounded corners. In various embodiments, the electrodes may have auniform area to match their signal generation capacity withcorresponding receiving capacity. In some cases, the diameter of theelectrodes relative to the distance between the centers of theelectrodes may vary. The Applicants have further discovered that theremay be a tradeoff between the electric field strength of the array, thegeometry factor of the array, and the signal-to-noise ratio of themeasurement obtained by the array. Applicants have further discoveredthat these factors are not determinant a priori to establish the optimumarea of the electrode.

Various approaches described allow for determining a physical propertyof one or more portions (e.g., sub-voxel or a number of sub-voxels) ofthe MUT 110. In various embodiments, a number of measurements of thephysical property(ies) of interest are measured by conventional meansand correlated with the measured variations of the measured (andcomputed) complex impedance (of the MUT, including one or more voxelsand sub-voxels) using the apparatuses/systems/approaches describedherein. In various embodiments, the number of measurements can besufficiently large such that the resulting correlation is statisticallysignificant. The impedance measurements can be made with the same typeof array that will be used to inspect unknown MUTs, or in otherembodiments, a parallel plate electrode arrangement may be used.Regardless of the array geometry, the measurements may also be made overa range of frequencies. Further embodiments include a method ofdeveloping an algorithm to correlate the physical property to themeasured impedance (of the voxel or sub-voxel over the selected range offrequencies), which may use any number of well-known correlation methodssuch as analysis of variations (ANOVA), neural networks, and multipleregressions. A determination as to which process, impedancecharacteristic(s) and frequency range may ensure that the best fit maybe made by selection of the one that provides the most statisticallysignificant results.

The functionality described herein, or portions thereof, and its variousmodifications (hereinafter “the functions”) can be implemented, at leastin part, via a computer program product, e.g., a computer programtangibly embodied in an information carrier, such as one or morenon-transitory machine-readable media, for execution by, or to controlthe operation of, one or more data processing apparatus, e.g., aprogrammable processor, a computer, multiple computers, and/orprogrammable logic components.

A computer program can be written in any form of programming language,including compiled or interpreted languages, and it can be deployed inany form, including as a stand-alone program or as a module, component,subroutine, or other unit suitable for use in a computing environment. Acomputer program can be deployed to be executed on one computer or onmultiple computers at one site or distributed across multiple sites andinterconnected by a network.

Actions associated with implementing all or part of the functions can beperformed by one or more programmable processors executing one or morecomputer programs to perform the functions of the calibration process.All or part of the functions can be implemented as, special purposelogic circuitry, e.g., an FPGA and/or an ASIC (application-specificintegrated circuit). Processors suitable for the execution of a computerprogram include, by way of example, both general and special purposemicroprocessors, and any one or more processors of any kind of digitalcomputer. Generally, a processor will receive instructions and data froma read-only memory or a random access memory or both. Components of acomputer include a processor for executing instructions and one or morememory devices for storing instructions and data.

In various embodiments, components described as being “coupled” to oneanother can be joined along one or more interfaces. In some embodiments,these interfaces can include junctions between distinct components, andin other cases, these interfaces can include a solidly and/or integrallyformed interconnection. That is, in some cases, components that are“coupled” to one another can be simultaneously formed to define a singlecontinuous member. However, in other embodiments, these coupledcomponents can be formed as separate members and be subsequently joinedthrough known processes (e.g., fastening, ultrasonic welding, bonding).

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a”, “an” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of embodiments are approximations, the numerical valuesset forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements. Moreover, all ranges disclosed hereinare to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5. In certain cases, the numerical values asstated for the parameter can take on negative values. In this case, theexample value of range stated as “less than 10” can assume negativevalues, e.g., −1, −2, −3, −10, −20, −30, etc.

When an element or layer is referred to as being “on”, “engaged to”,“connected to” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto”, “directly connected to” or “directly coupled to” another element orlayer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

Spatially relative terms, such as “inner,” “outer,” “beneath”, “below”,“lower”, “above”, “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. An apparatus comprising: a planar sensor array ofat least two electrodes configured to be placed in electromagneticcommunication with a material under test (MUT), wherein during operationof the apparatus, at least one of the electrodes comprises atransmitting electrode for transmitting an electromagnetic signal over arange of frequencies through the MUT to at least one receiving electrodein the planar array; and a conductive backer ground plate at leastpartially surrounding the at least two electrodes, the conductive backerground plate being electrically grounded and insulated from the at leasttwo electrodes, wherein the conductive backer ground plate extends froma plane formed by the at least two electrodes and separates the at leasttwo electrodes to create an electrically isolated volume proximate toeach of the at least two electrodes.
 2. The apparatus of claim 1,wherein the conductive backer ground plate extends between neighboringelectrodes in the sensor array and defines a set of separate volumescorresponding with each of the at least two electrodes.
 3. The apparatusof claim 2, wherein the plane formed by the at least two electrodes andthe conductive backer ground plate is substantially parallel with asurface of the MUT.
 4. The apparatus of claim 3, wherein the array of atleast two electrodes comprises axisymmetric circular electrodes, whereina first one of the at least two electrodes comprises a central electrodeand a second one of the at least two electrodes comprises an outerelectrode.
 5. The apparatus of claim 4, wherein the central electrodecomprises the transmitting electrode and wherein the outer electrodecomprises the receiving electrode.
 6. The apparatus of claim 4, whereinthe central electrode comprises the receiving electrode and wherein theouter electrode comprises the transmitting electrode.
 7. The apparatusof claim 1, wherein the planar sensor array is arranged in a linearrectangular configuration, wherein a first one of the at least twoelectrodes is a transmitting electrode and a second one of the at leasttwo electrodes is a receiving electrode.
 8. The apparatus of claim 1,wherein a spacing of the conductive backer ground plate between thetransmitting electrode and receiving electrode is sized to adjust for adepth of penetration and measurement into the MUT.
 9. The apparatus ofclaim 1, wherein each of the at least two electrodes has a rectangularshape with rounded corners.
 10. The apparatus of claim 1, wherein thearray of at least two electrodes comprises at least three electrodesarranged in a linear rectangular configuration.
 11. The apparatus ofclaim 1, wherein a single electrode in the planar sensor array comprisesthe transmitting electrode and the remaining electrodes in the planarsensor array comprise the at least one receiving electrode.
 12. Theapparatus of claim 1, wherein the planar sensor array comprises aplurality of transmitting electrodes and a plurality of receivingelectrodes.
 13. The apparatus of claim 1, wherein the transmittingelectrode is selectable among the electrodes of the planar sensor array,and is configured to change between the electrodes of the planar sensorarray.
 14. The apparatus of claim 1, wherein during operation of theapparatus, the planar sensor array is in direct physical contact withthe MUT and is either electrically non-conductive with the MUT orelectrically conductive with the MUT.
 15. A system comprising: anapparatus for communicating with a material under test (MUT), theapparatus comprising: a planar sensor array of at least two electrodesconfigured to be placed in electromagnetic communication with the MUT,wherein during operation of the apparatus at least one of the electrodescomprises a transmitting electrode for transmitting an electromagneticsignal over a range of frequencies through the MUT to at least onereceiving electrode in the planar sensor array; and a conductive backerground plate at least partially surrounding the at least two electrodes,the conductive backer ground plate being electrically grounded andinsulated from the at least two electrodes, wherein the conductivebacker ground plate extends from a plane formed by the at least twoelectrodes and separates the at least two electrodes to create anelectrically isolated volume proximate to the at least two electrodes;and a circuit coupled with the apparatus, the circuit configured totransmit a set of electromagnetic signals through the planar sensorarray through the MUT, and receive at least one return signal from theplanar sensor array.
 16. The system of claim 15, wherein the circuitincludes a signal generator operating over a range of frequencies fortransmitting the electromagnetic signal from the transmitting electrodeof the planar sensor system through the MUT and a signal analyzerconfigured to analyze the at least one return signal.
 17. The system ofclaim 16, wherein the circuit further includes a computing devicecoupled with the signal analyzer, wherein the computing device isconfigured to determine a physical property of the MUT based upon thetransmitted electromagnetic signal and the at least one return signal.18. The system of claim 17, wherein determining the physical property ofthe MUT comprises: transmitting the electromagnetic signal over a rangeof frequencies from the transmitting electrode through the MUT to thereceiving electrode; comparing the transmitted electromagnetic signal tothe received signal to compute an electromagnetic property of the MUT;and correlating the computed electromagnetic property of the MUT with aphysical property of the MUT to determine the physical property of theMUT.
 19. The system of claim 15, wherein during operation of the system,the planar sensor array is in direct physical contact with the MUT andis either electrically non-conductive with the MUT or electricallyconductive with the MUT.